The resolution and optical isomers of 6-dimethylamino-4, 4-diphenyl-5-methyl-3-hexanone



United States Patent C THE RESOLUTION AND OPTICAL ISOMERS F 6-DIMETHYLAlml0-4A-DlPI-IENYL-5- METHYL-S-IEXANONE Aubrey A. Larsen and Benjamin F. Tullar, Rensselaer County, N. Y., assignors to Sterling Drug Inc., Wilmington, Del., a corporation of Delaware N0 Drawing. Application March 16, 1948, Serial N0. 15,267

1 Claim. (Cl. 260-5703) This invention relates to the optically active stereoisomers of o-dimethylamino-4,4-diphenyl--methyl-3-hexanone and their water soluble salts.

An isomer of 6-dimethylamino-4,4-diphenyl-5-methyl 3-hexanone, namely, 6-dimethylatnino-4,4-diphenyl-3-heptanone, was recently synthesized in Germany and found to be a potent analgetic agent (Office of the Publication Board Report No. 981, p. 91, July 1945). More recently, the preparation of racemic 6-dimethylamino-4,4- diphenyl-5-methy1-3-hexanone has been reported [Schultz, Robb and Sprague, I. Am. Chem. Soc. 69, 188 and 2454 (1947); Thorpe, Walton and Ofner, Nature 160, 605 (1947); Easton, Gardner and Stevens, 1. Am. Chem. Soc. 69, 976 (1947)].

Our invention consists in preparing the optically active enantiomorphic forms of 6-dirnethylamino-4,4-diphenyl- 5-methyl-3-hexanone. It has been found that the levoform retains the large majority .of the activity present in the racemate, whereas the dextroform is relatively inactive. This achievement means that the physiological activity of this substance has been enhanced, rendering the preparation considerably more potent with no increase in toxicity. The levo-form of 6-dimethylamino-4,4di phenyl-5-1nethyl-3-hexanone is ordinarily used in the form of water-soluble salts, derived from inorganic or organic acids, the anions of which are nontoxic and otherwise innocuous to the animal organism at the dosage levels required for therapeutic results. Examples of such acids include hydrochloric acid, hydrobromic acid, hydriodic acid, sulfuric acid, citric acid, tartaric acid, etc.

When diphenylacetonitrile is condensed with l-dimethylamino-Z-chloropropane in the presence of a base such as sodamide or potassium tertiary-butoxide, the major portion of the product consists of 2,2-diphenyl-4- dimethylaminopentanenitrile which can arise only by molecular rearrangement in the alkamine' side-chain. The remainder, about 20-50% of the total product, is the normal condensation product, 2,2-diphenyl-3-methyle'4- dimethylaminobutanenitrile. of 6-dimethylamino-4,4-diphenyl-5-methyl-3 hexanone. This latter compound is obtained by reaction of. the nitrile with ethylmagnesium bromide, followed by hydrolysis.

The optically active isomers of 6-dimethylamino-4,4- diphenyl-5-methyl-3-hexanonc may be obtained by either of two methods starting from racemic 2,2-dipheny1-3- methyl-4-dimethylaminobutanenitrile. The racemic nitrile (I) may be resolved, using dextro-tartaric acid into the levo-nitrile and dextro-nitrile. The latter are then respectively converted by the Grignard reaction into the dextro-ketone and levo-ketone. A reversal of sign in the optical rotation is observed upon conversion of the optically active nitriles to the optically active ketones. Alternatively, the racemic nitrile (I) may be converted-by the Grignard reaction into the racemic ketone (II), which in turn is resolved with dextro-tartaric acid into the dextro- The latter is the precursor 2,773,901 Patented Dec. 1 1, 1956 ketone and. the levo-ketone. The following reaction chart illustrates the processes involved:

a Resolution dextro- Ievo: (GHs)zN-=OH:(]JH-CCN dt nitrile nitrile ex ros CH3 05H tartaric acid (1) zHsMgB zHs (2) Hydrolysis (2) Hydrolysis CaHs Resolution levodextro- (CHa) 2NCHzCHC-C O C2115 ketone ketone l I dextro- 1 CH3 OsH tartaricacid The solvent used' for the resolutions is water or aqueous ethanol. The racemic nitrile is resolved by'reacting it with slightly more than one equivalent of dextro-tartaric acid in ethanol, giving in solution a mixture of the diastereoisomeric salts, dextro-nitrile dextro-bitartrate and levo nitrile dextro-bitartrate. The former crystallizes preferentially upon cooling and is carried out most readily by seeding the solution with a trace of crystalline dextro-nitrile dextro bitartrate. Fractional crystallization permits nearly complete separation of the two isomeric salts which precipitate in the hydrated form. The optically active nitriles themselves are produced by addition of strong base to solutions of the diastereoisorneric salts.

The dextro-bitartrate salts of the levoand dextroket'ones are more soluble than those of the nitriles and water may be used as the resolution medium. In this case the levo-ketone dextro-bitartrate crystallizes preferentially.

It is also possible to use levo-tartaric acid for these resolutions. In this case the corresponding enantiornorphic salts crystallize preferentially, namely the levo-nitrile levo-bitartrate and the dextro-ketone levo-bitartrate. However, this result is undesirable since it leads to a majority of the undesired isomer. Moreover the use of dextro-t'artaric acid is more economical.

Example 1 (a) Purification of 2,2-diphenyl-3-methyl-4-dimethylaminobutanenitrile.-This is obtained from the condensation of diphenylacetonitrile and 1-dimethylamino-2- chloropropane, after removal of the large majority of 2,2-

diphenyl-4-dimethylaminopentanenitrile, which is the mixture is cooled to 5- C. for one hour, and the product is filtered OE and washed well with isopropanol and then with ether and dried at 60 C. for 24 hours; yield 4 kg. of nitrile hydrochloride, M. P. 223-225 This crop is dissolved in 35 liters of water, andthe free nitrile base is crystallized out by slow addition of 1200 cc. of am-- monium hydroxide after seeding. The base is filteredf off, washed well with water and dried. The product is: crystallized from 7 liters of isopropyl alcohol, yielding: 3.3 kg, M. P. 67-68 C. By concentration a second crop of 0.23 kg. is obtained, which gives about 200 g.,'M. P 67-68' C. upon recrystallization.

The hydrochloride liquor is concentrated in vacuo to chloride. After a crop of this has been removed as the free base, it is possible, by-distillation of theresidual oil and removal of the low boiling fraction, to isolate an-- other 150 g. of pure 2,2-diphenyl-3-methyl-4-dimethylaminobutanenitrile hydrochloride.

. (17) Resolution of racemic 2,2-diphenyL3-methyl-4- dimethylaminobutanenitrile.-Two kg. of racemic 2,2- diphenyl 3-methyl 4 dimethylaminobutanenitrile and 1120 g. of dextro-tartaric acid are dissolved in 15 liters of 95% ethyl alcohol. seeded with a trace of the dextro-nitrile dextro-bitartrate if available, and allowed to stand at 5 C. for two days. Rapid crystallization without seeding can be obtained by stirring, but a precipitate is produced which filters very slowly, and the resolution will not be as effective. The solid is collected by filtration, washed with 2 liters of cold 95% alcohol and dried as thoroughly as possible on thesuction filter. A sample converted to the free dextro-nitrile base has the M. P. 8292 C. The crude salt is then dissolved in 4 liters of 95 alcohol at 50 C., cooled rapidly to 5 C. and kept at this temperature for 24 hours. The resulting precipitate is collected by filtration, washed with cold 95 alcohol and dried in vacuo to constant weight, giving about 1220 g. of dextro-2,2- diphenyl-3-methyl-4-dimethylaminobutanenitrile dextrobitartrate, M. P. 75-95" C.; a =+63.8(l.6% in H20).

The recrystallized dextro-nitrile dextro-bitartrate is converted to the dextro-nitrile by dissolving the salt in 15 liters of water and treating with ammonium hydroxide. The solution is seeded with a sample of the dextro-nitrile at the point of incipient turbidity, and the addition of ammonium hydroxide is finished slowly with good stirring to eflect good crystallization. The base is separated, washed well with water, dried and recrystallized from 2 liters of 95% alcohol, yielding about 610 g. of pure dextro-2,2-diphenyl 3 methyl-4-dimethylaminobutanenitrile, M. P. 10l.2-lO2.2 C. [oz =+70]. This corresponds to 61% of theory for the first phase of the resolution. 2

Additional amounts of the dextro-nitrile dextro-bitartrate are obtained by working up the combined resolution liquors as follows. When the combined mother liquors are allowed to stand at 25 C. for several days (or quite rapidly if seeded), a heavy crop of levo-2,2-

diphenyl-3-methyl-4-dimethylaminobutanenitrile dextrobitartrate is obtained. This solid is collected by filtration, washed with a little 95% alcohol and dried, giving about 690 g. or 40% of the theoretical amount of the levonitrile dextro-bitartrate of about 95% purity. The filtrate is concentrated to l-l2 liters and allowed to stand at 25 C. for 24 hours, whereupon a second crop of the levo-isomer separates, amounting'to 13% of the theoretical. When the filtrate from this is cooled to 5 for 24 hours, 520 g. of the crude dextro-isomer separates which by recrystallization gives a 20% recovery of the pure dextro-salt.

(c) Lev0-6-dimethylamino 4,4 diphenyl S-methylhexanone-3 h'ydrobromide.Dextro 2,2 diphenyl 3- methyl-4-dimethylaminobutanenitrile is converted into pure levo-6-dimethylamino 4,4-diphenyl-5methylhexanone-3 hydrobromide in about 71.5% yield by treatment with ethyl magnesium bromide as described below in Example 2 for the conversion of the racemicnitrile to the racemic ketone. The dextro-nitrile (610 g.) yielded about 613 g. of pure levo-6-dimethylamino-4,4-diphenyl- 5-methyl-3-hexanone hydrobromide, M. P. 217218 C. [m =59(l.5% in H2O)l.

(d) Levo-fi-dimethylamino 4,4 diphenyl-5-methyl-3- hexanone hydrochloride.-The hydrobromide prepared in part (c) is converted quantitatively to the free levoketone base and this in turn to the hydrochloride as described below in Example 2 (part c.) for the conversion of levo-fi-dimethylamino 4,4 diphenyl-5-methyl-3'hex The solution is cooled to 5 C.,

4 anone dextro-bitartrate .to the hydrochloride. The levo ketone hydrobromide (603 g.) yields about 480 g. of levo-ketone base, which in turn is converted into about 526g. or 95 of theory from the hydrobromide, of pure levo-6-dirnethylamino-4,4-diphenyl 5 -methyl-3-hexanone hydrochloride. The product thus obtained contains about 3.8% of moisture which can be removed by heating at 90l00 C. for 12-15 hours in vacuo. M. 229.523l.5 C. [a ='-70 (1.6% in H2O)l.

In an attempted racemization of a sample of the levoketone hydrobromide by six hours of refluxing in constant boiling hydrochloric acid, a nearly quantitative recovery of pure levoketone hydrochloride was observed. Thus it is unnecessary to proceed through the base to prepare the hydrochloride from the hydrobromide.

Example 2 6 dimethylamino 4,4 diphenyl 5- hydr0br0mide.-Into a twelve- (a) Racemic m'ethyl 3 hexanone liter, three-neck flask equipped with a stirrer, dropping funnel, thermometer and condenser fitted with a drying tube, is placed 348 g. of magnesium turnings and- 2250 cc. of ether which has been dried over sodium. A solution of 1560 g. (1090 cc.) of ethyl bromide in 750 cc.

of dry ether is then slowly introduced through the dropstirring is continued for an additional 45 minutes to dissolve practically all of the remaining magnesium.-

- A solution of 1 kg. of racemic 2,2-diphenyl-3-methyl- 4-dimethylaminobutanenitrile in 1500. cc. ofdry toluene I time practically all of the toluene boils off.) Hydrolysisis then added in a steady streamover a period of two to three minutes with stirring. After distilling-off the ether until an internal temperature of 106l08 C.is reached,.

the mixture 'is refluxed for two and one-half hours.

Hydrolysis is accomplished by carefully pouring the toluene suspension into a stirred solution of 4 liters of concentrated hydrochloric .acid in 8750 cc. of water. (This must be carried out under an effective hood since a rather violent decomposition takes place during which is completed byheating the resulting acidic solution under reflux for six hours. a I

The crude hydrobromide is isolated by cooling the hydrolysis mixture to about 5 C. in an ice bath while .maintaining a slow rate of stirring. The light gray .has the M. P. l43l48 C.

crystalline solid is collected by filtration, washed with a 6-dimethylamin0-4,4-diphenyl-5-methyl-3-hexanone bydrobromide are formed which are collected on a suction funnel, washed with a small amount of cold water and pressed as dry as possible. The remaining moisture is removed by drying overnight in a vacuum chamber, giving about 980 g., M. P. 145-148 C., 70% of theory. A second'crop isobtained by concentration of, the

mother liquorsto about 500 ,cc., cooling, and filtering. This material is combined with a small amount of crude material which gradually separates from the hydrolysis mother liquor on standing, and is recrystallized from water. After drying, it weighs about g. and melts-at l49l50 C. This represents a total yield of about 1075 g. or,76.5% of theory. a

(b) The resolution of racemic 6-dimethylamin0-4,4- diph'enyl-S-methyl-3-hexanone. Racemic 6-dime'thyl amino-4,4-diphenyl-5-methyl-3-hexanone After air drying, the solid It.

After cooling to 5 C. in an ice bath, white crystals of racemic hydrobromide (980 g.), M. P. 145-148 C., is dissolved in 7 liters of water at 70 C., made alkaline with 230 cc. of ammonium hydroxide, and cooled to 25 C. The viscous oily base is then extracted with 3 liters of ether. The extract is freed of ether and traces of water finally in vacuo, giving 780 g. of light yellow viscous oil. This oil is the free base, racemic 6-dimethylamino-4,4-diphenyl-5-methyl-3- hexanone, 760 g. of which is then dissolved in 2 liters of water along with 390 g. of dextro-tartaric acid and diluted with 6 liters of water. A trace of levo-6-dimethylamino- 4,4 diphenyl 5 methyl 3 hexanone dextrobitartrate is then added, and let stand for 24 hours at 20 C. while a heavy crop of crystals separates. This is collected by filtration and sucked as dry as possible on the filter. Without further drying this is dissolved in 3 liters of water and kept at 22 C. overnight. The glistening crystals are filtered E and dried as thoroughly as possible. A sample dried for four hours at 80 C. has an indefinite melting point and a =39 (1.7% in H2O). The whole product is dissolved in 2 liters of water at 60-70 C. and allowed to stand at 22 C. for six to eight hours. The precipitate is separated, sucked dry, and dried in vacuo finally at 100 C. (8-10 mm.) for four hours. There is obtained 346 g. (60% of theory) of levo- 6-dimethylamino-4,4-diphenyl-5-methyl-3-hexanone dextro-bitartrate, M. P. l15-l20 C., a =-43.

A further 20% of the theoretical amount of levo isomer is readily recovered by the following procedure. The combined resolution liquor and first recrystallization liquor are treated with 500 cc. of concentrated hydrochloric acid. The crystalline precipitate which separates during one to two hours at room temperature is filtered 01f, washed with a little dilute hydrochloric acid and ice water, and dried. In this way nearly 300 g. of pure dextro- 6-dimethylamino-4,4-diphenyl-5-methyl-3-hexanone hydrochloride is obtained, u =+64 to +66. The filtrate from the dextro-ketone is saturated with sodium chloride and cooled to 0 C., giving a second crop of hydrochloride which is slightly levo-rotating (a =-10) and amounting to 240 g.

The second tartrate recrystallization liquor, when treated as above with 200 cc. of concentrated hydrochloric acid, gives, as a first crop, 35 g. of nearly pure levo-6-dimethylamino-4,4-diphenyl -5- methyl-3 -hexanone hydrochloride, u =-66. Addition of sodium chloride to the filtrate gives an additional 30 g. of what is essentially racemic 6-dimethylamino-4,4diphenyl-5-methyl-3-hexanone hydrochloride, which with the above 240 g. of slightly levo-rotating material obtained above, is converted to the free base and the latter submitted to a second resolution yielding an additional 70 g. of pure levo-6-dimethylamino-4,4-diphenyl-5-methyl -3- hexanone hydrochloride. In this way, and by conversion of the bitartrate to the hydrochloride as given below in part (c), 80% of the theoretical amount of levo-ketone is obtained. Repetition of the steps outlined gives another 10%.

(c) Levo-6-dz'methylamina-4,4-diphenyl -5- methylhexmane-3 and its hydr0chl0ride.-A solution of 336 g. of levo-ketone dextro-bitartrate in 2 liters of water is made alkaline to phenolphthalein with ammonium hydroxide. The base is extracted with ether, the ether removed by distillation, and the product is dried in vacuo for one hour at 100 C., giving 220 g. of levo-6-dimethylarnin0- 4,4-diphenyl-5-methyl-3-hexanone. The levo-ketone base (210 g.) is dissolved in 500 cc. of water and 60 cc. of concentrated hydrochloric acid at 90 C. The solution is filtered, and the flask and filter are rinsed with 200 cc. of boiling water. Upon cooling the combined filtrates to 5 C., a crystalline precipitate forms, which is collected by filtration, washed with 50 cc. of ice water, with 200 cc. of acetone and finally with ether. After fifteen hours at 90 C. (2 mm.), 223 g. or 95% of theory of white crystalline levo-6-dimethylamino-4,4-diphenyl-5- methyl-3-hexanone hydrochloride is obtained. The product still contains 0.5% of moisture and has the M. P. 231-233 C. with softening at 160 C., whereas the anhydrous salt exhibits no sintering below the true melting point; a =70 (1.5% in H2O).

A second recrystallization of the bitartrate and conversion to the base is not necessary. If the once-recrystallized bitartrate is dissolved in 4-5 liters of water and acidified to Congo Red with hydrochloric acid, 60-65% yield of pure levo-hydrochloride crystallizes out at room temperature.

The following table summarizes some physical constants of resolved 2,2-diphenyl-3methyl-4-dimethylami nobutanenitrile and 6 dimethylamino 4,4 diphenyl-5- methyl-3-hexnone, and some of their water-soluble salts.

We claim:

The process of preparing levo-6-dimethylamino-4,4- diphenyl-5-methyl-3-hexanone which comprises reacting racemic 2,2 diphenyl-3-methyl-4-dimethylaminobutanenitrile of the formula with at least one equivalent of dextro-tartaric acid to form a mixture of diasteroisomeric salts, separating the dextro 2,2 diphenyl 3-methyl-4-dimethylaminobutanenitrile dextro-bitartrate from said diastereoisomeric salt mixture by fractional crystallization, liberating the free dextro-nitrile by reacting said dextro-2,2-diphenyl-3- methyl-4-dimethylaminobutanenitrile dextro-tartrate with an excess of a strong base, reacting said dextro-nitrile with ethyl magnesium bromide, and hydrolyzing the resulting Grignard complex by heating it with mineral acid.

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Brode et al.: Am. Chem. Soc., Abst. of Papers of 112th Meeting, Sept. 17, 1947, pp. 14K and 15K.

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